SPID#: 51 To determine if the presence of familiar peers buffers the physiological response to social stress, ten juvenile rhesus monkeys were removed from natal groups and placed together to form a new peer group. The subjects were selected such that six came from groups with one or more companions and four came individually from separate social groups. Six age matched subjects remained in natal groups as controls. Blood samples were obtained from all subjects prior to group formation and at 24 hr, one week and four weeks following formation of the peer group, and were assayed for cortisol concentration and lmphocyte subsets. There was a significant interaction among the three study groups and sample times for T-helper, T-suppressor and B-lymphocytes, but not cortisol. Subjects introduced into the peer group showed significant declines in lymphocyte subsets compared to controls, with companion subjects demonstrating a significantly smaller decline than subjects without natal group companions. Planned post-hoc comparisons also revealed a significant difference in cortisol concentrations at the one week sample, with companion subjects having lower levels than those without companions. These data extend the growing body of human and nonhuman primate literature documenting that social support can modulate the endocrine and immunological response to social stress. The results further our understanding of the links between stress, social support and health.